This invention relates to systems that are used by paper mills, and, more specifically, to a black liquor gasification system through the operation of which by using a fluidized bed reactor, which makes a dry product, chemicals from a Kraft process can be recovered without forming a smelt thereby removing the potential for smelt/water explosions.
The type of systems to which reference is made herein historically have been comprised of what is commonly known in the industry as a Chemical Recovery Unit or CRU. Chemical Recovery Units are boilers that are used by paper mills for two reasons. The primary reason, as the name Chemical Recovery Unit suggests, is to recover the chemicals that are used in the process of breaking down the wood into the pulp which is then made into paper. The wood in the process of being broken down is treated in a vessel called a digester by heating with steam and mixing with chemicals. The used chemical residue which is produced as a result of processing the wood in the digester is commonly referred to as "Weak Black Liquor". This liquid is mainly water, i.e., approximately 84%, at this stage. Subsequent processing reduces the water content until the water content is approximately 32% of the volume. The liquid in this state is commonly referred to as "Strong Black Liquor". "Strong Black Liquor" constitutes the fuel which is burned in the Chemical Recovery Unit, i.e., boiler, to make steam. As the "Strong Black Liquor" is burned in the Chemical Recovery Unit the chemicals which do not burn form a molten pool in the bottom of the Chemical Recovery Unit and this pool, referred to in the industry as the "smelt", is continuously drawn off and processed to return the chemicals to the digester whereupon the cycle is once again repeated.
Chemical Recovery Units insofar as they are boilers operate in essentially the same manner as any other boiler. As do all other boilers, Chemical Recovery Units burn a fuel to create heat. This heat in turn is used to convert water into steam which is used either as steam or converted via the use of a turbine into electricity. The one main difference between a Chemical Recovery Unit and all other types of boilers is that the Chemical Recovery Unit unlike all other types of boilers has at all times in the bottom thereof a smelt bed.
The potential result of a waterwall leak in any other type of boiler other than a Chemical Recovery Unit is the loss of pressure and the destruction of a few of the surrounding tubes. On the other hand, in a Chemical Recovery Unit if the water from the waterwall should contact the smelt bed the potential exists for a violent explosion. While the typical explosions experienced in a boiler which uses a conventional fuel such as coal, oil or gas in air produces high pressures and travels at the speed of sound, smelt-water explosions produce much higher pressures, perhaps by a factor of 10 or more, and travel at supersonic speeds. A smelt-water explosion is the result of water changing suddenly to steam. The rapid change in state is the result of an extreme heat transfer rate, and the seriousness of a smelt-water explosion is not directly related to the amount of smelt or water involved. A smelt-water explosion is one of the major personnel safety issues in the paper mill. In addition this also leads to higher than desired insurance premiums for the plant. As such, the paper companies are very desirous of having new and improved systems developed that would enable them to produce paper in a manner which is safer and more cost effective.
One prior art form of Chemical Recovery Unit comprises the subject matter of U.S. Pat. No. 3,047,362 entitled "Treatment of Waste Liquors", which issued on Jul. 31, 1962 and which is assigned to the same assignee as the present application. In accordance with the teachings of the aforereferenced U.S. Pat. No. 3,047,362, the nature of the construction and the mode of operation of the Chemical Recovery Unit that is described and illustrated therein is as follows. The Chemical Recovery Unit has a furnace that extends upwardly and has as its upper end a boiler. Concentrated liquor is introduced into the furnace through spray nozzles with the liquor being dried almost instantaneously as it passes down toward the bottom of the furnace. The burnables of the liquor are being burned during this descent. The chemicals in the liquor, on the other hand, are smelted in the furnace and collect at the bottom thereof with these chemicals being continuously withdrawn through a suitable spout. The combustion gases generated by burning the burnables of the liquor in the furnace pass upwardly over the tubes of the boiler and leave the boiler at the outlet thereof. These gases which have a relatively high temperature, as for example, 700.degree. to 750.degree. F., pass through an air heater. In passing through this air heater a large portion of the remaining heat in the combustion gases is removed with these gases afterward leaving the air heater, passing through an induced draft fan and to the stack for discharge to the atmosphere and with the temperature of the gases having been reduced by the air heater so that they may be so discharged without excessive waste of heat.
With further reference to the teachings to be found set forth in U.S. Pat. No. 3,047,362, the liquor as it comes from the pulp digester has a concentration of about 15% solids. This liquor is concentrated to about 55% solids by means of steam that is supplied from the boiler of the Chemical Recovery Unit. Thereafter, the liquor is concentrated to its desired concentration, as for example, from 55% solids to a concentration of between 65 and 75% solids by being subjected to air from the air heater, which has been referred to herein previously. After having been concentrated to its desired concentration of between 65 and 75%, the liquor is then introduced into the furnace of the Chemical Recovery Unit through the spray nozzles to which reference has been had hereinbefore.
Efforts have been made previously in an attempt to minimize the difficulties occasioned by the existence of smelt in a Chemical Recovery Unit. Some of these efforts are acknowledged in U.S. Pat. No. 4,872,950 entitled "Process for Recovering Energy and Chemicals from Spent Liquor in Pulp Preparation" that issued on Oct. 10, 1989. For example, in accordance with one such attempt which is acknowledged therein, i.e., in the Champion process (TAPPI Journal, Nov. 1985, pages 106-110), concentrated black liquor is injected at the upper part of a vertical furnace and is thermally decomposed (gasified) under access of air forming a melt and a combustible gas. The melt is collected at the bottom of the furnace and discharged to the recovery of chemicals, and the gas is burned in a gas turbine or a steam boiler.
Another such attempt, which is also acknowledged in U.S. Pat. No. 4,872,950, involves the SCA-Billerud process (E. Hornstedt and J. Gommi, Paper Trade Journal 158 (1974):16, pages 32-34) wherein the liquor is subjected to pyrolysis in a reactor under such temperature conditions that a dust substantially consisting of sodium carbonate and carbon is obtained as well as a combustible gas containing sulfur compounds. The solid and gaseous materials are separated in a wet scrubber at the same time as the sulfur-containing gases are being absorbed by the aforementioned sodium carbonate solution that has been formed. The remaining gases are then burned in a boiler to recover energy therefrom while the carbon is removed by means of filtration from the resulting liquid phase that is obtained from the scrubber. Thereafter, this liquid phase is subjected to causticizing in the usual manner in order to obtain white liquor.
A third such attempt, which is also acknowledged in U.S. Pat. No. 4,872,950, is that process for recovery of energy and chemicals in a sulfate process which can be found described in the international patent application PCT/SE86/00249. In accordance with this process, the concentrated black liquor is gasified in a pressurized reactor by means of so-called "flash pyrolysis" at 700.degree.-1300.degree. C. whereby an energy-rich gas and a melt of substantially sodium carbonate and sodium sulfide are formed. This melt is directly dissolved in water and the solution resulting therefrom may be used for washing out hydrogen sulfide from the gaseous fraction. Thereafter, the green liquor thus formed is recycled for use in the preparation of the digesting liquor. The gas in turn is used as the fuel which is burned for purposes of generating steam.
Notwithstanding the efforts that have been made heretofore, as exemplified by those efforts to which specific reference has been made hereinbefore, the problems associated with the formation of the melt, i.e., the smelt, in the Chemical Recovery Unit were not eliminated. Moreover, to the extent that the formation of the melt were minimized, as in the case of the SCA-Billerud process to which reference has been had hereinbefore, another problem was created. Namely, the steps taken, such as in the SCA-Billerud process, to minimize the formation of the melt had the concomitant undesirable effect of resulting in an increase in the formation of carbon. Accordingly, it was believed that there still existed a need for a new and improved process, the use of which would neither result in the formation of a melt, i.e., a smelt, nor at the same time would there be a resulting increase in the formation of carbon. The process, which comprises the inventive subject matter of U.S. Pat. No. 4,872,950 entitled "Process for Recovering Energy and Chemicals from Spent Liquor in Pulp Preparation", and which issued on Oct. 10, 1989, is indicated to be such a process.
In accordance with this process, as described and illustrated in the aforementioned U.S. Pat. No. 4,872,950, a concentrated spent liquor from the preparation of pulp is thermally decomposed forming gaseous and solid products. This thermal decomposition is carried out with a supply of oxygen in an amount below the stoichiometrically required amount, at a pressure above atmospheric and at such a temperature that no melt is formed. To this end, it has been found, according to the teachings of the aforementioned U.S. Pat. No. 4,872,950, that by gasifying the black liquor at an elevated pressure of 10-50 bar and preferably at 15-25 bar, it is possible to operate at higher temperatures and still obtain the main portion of the included sulfur in the form of gaseous hydrogen sulfide. At the same time, the formation of carbon from the included organic materials is suppressed due to the higher working temperature. Obviously, however, the working temperature must not be raised so high as to cause a melt to form.
To the extent that it is possible with the process, which comprises the inventive subject matter of the aforementioned U.S. Pat. No. 4,872,950, to avoid the formation of a melt while at the same time the formation of carbon is being suppressed, this process is nevertheless disadvantageously characterized in other respects. For example, the process of U.S. Pat. No. 4,872,950 requires for its implementation that it be carried out at elevated pressures. However, it is well-known that the need for a process to be operated at elevated pressures introduces additional complexity to the process. In addition to being inherently more complex, those components employed in such a process that are required to operate at elevated pressures are normally more costly to provide than are components which operate essentially at atmospheric pressure or at slightly higher pressure levels, e.g., on the order of 1 to 2 bar. Moreover, there generally is a need to take additional safety concerns into account when pressurized components are involved than when unpressurized components are involved.
Although processes of the type acknowledged in the aforementioned U.S. Pat. No. 4,872,950 as well as the process which comprises the inventive subject matter of U.S. Pat. No. 4,872,950 itself have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such processes to be further improved if the paper companies are to be able to produce paper in a manner which is safer and more cost effective than that presently being utilized by the paper companies. Namely, a need is being evidenced in the prior art for a new and improved process which would be capable of serving as a replacement for a Chemical Recovery Unit. Moreover, there has been evidenced in the prior art a need for such a new and improved process that would be characterized in a number of respects. To this end, one such characteristic which such a new and improved process would desirably possess is that in the operation thereof no smelt would be formed. Another characteristic which such a new and improved process would desirably possess is that in the operation thereof there would be no increased formation of carbon. A third characteristic which such a new and improved process would desirably possess is the capability of producing dry recoverable salts. A fourth characteristic which such a new and improved process would desirably possess is the capability of producing low calorific gases to be used within the paper-making process. A fifth characteristic which such a new and improved process would desirably possess is that it be inherently safer than the processes which are presently being utilized for similar purposes. A sixth characteristic which such a new and improved process would desirably possess is that it be more cost effective than the processes which are presently being utilized for similar purposes. A seventh characteristic which such a new and improved process would desirably possess is that it be at least no more difficult to operate than the processes which are presently being utilized for similar purposes.
To thus summarize, a need has been evidenced in the prior art for such a new and improved process that would be particularly suited for use by paper companies as a replacement for Chemical Recovery Units. Moreover, the usage of such a new and improved process would be accomplished in such a manner as to preclude smeltwater reactions and explosions by virtue of the fact that the temperature which is employed in the process is kept below the ash melting temperature. Furthermore, the usage of such a new and improved process would be effected such that there are produced therewith for utilization within the paper-making process both dry, recoverable salts and low calorific gases.
It is, therefore, an object of the present invention to provide a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit.
It is a further object of the present invention to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that in the operation thereof no smelt is formed.
It is another object of the present invention to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that in the operation thereof there is no increased formation of carbon.
Another object of the present invention is to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that dry recoverable salts that are capable of being utilized in the paper-making process are produced therewith.
Still another object of the present invention is to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that low calorific gases that are capable of being utilized in the paper-making process are produced therewith.
A further object of the present invention is to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that it is inherently safer than the systems which are presently being utilized for similar purposes.
Yet another object of the present invention is to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that it is more cost effective than the systems which are presently being utilized for similar purposes.
Yet a further object of the present invention is to provide such a new and improved system for use as a replacement for a Tomlinson cycle, Chemical Recovery Unit which is characterized in that it is at least no more difficult to operate than the systems which are presently being utilized for similar purposes.